Australia is in the grip of its most severe drought on record, and severe water restrictions are in place in major cities on the eastern seaboard. Warragamba Dam provides 90 per cent of Sydney’s water, but 10 per cent of this is inaccessible because it is below the water inlet. (See also ‘Multi-level lifting’ for another recent job from this site, CT October, p. 31-5).

The block comes back up

The 160-tonne crane raising the 17-tonne concrete block inside the specially developed cage

In a bid to access this water, the New South Wales Government funded a world-first deep water access project to cut a lower inlet into the wall of the dam. As part of this it was necessary to cut a 17-tonne block of concrete two metres square and 1.8 metres deep from the wall 80 metres below the surface of the water, and raise it to the dam wall.

The many constraints on this project made the lift a much more difficult exercise than would first be imagined, and involved the devising of a range of equipment, some of it representing a world first.

A number of the subcontractors for the project, including the divers (Covus) and equipment suppliers, builders and operators (Jetcut), came from the offshore oil industry as this was the closest parallel to the Warragamba Dam project conditions.

The job

A diagram of the underwater job, showing the yellow main frame, the hole that was cut (in grey), and the lifting spreader in red

The cornerstone of the operation was a universal main frame that was lowered into a narrow alcove in the dam wall, and hydraulically anchored to the wall (it was guided by divers but controlled from a Launch and Recovery Station [LARS] situated on the dam wall, and linked to the work area by a fibre optic umbilical cord).

The frame served as a platform for tool modules that were lowered from the surface and used to core drill the wall, wire saw between the cores to cut the block, insert bearing packs to allow the block to be moved, and extract the block into the main frame which them served to support the block while it was lifted out of the water.

Saturation diving allowed divers to work below the surface, in three teams of two. Divers transferred between the water and a pressurising chamber on top of the dam wall using a diving chamber that was raised and lowered using a gantry attached to the chamber. As much as possible of the operations in the water was automated, to minimise the demands on the divers. The divers worked around the clock for a two weeks to complete the work, and took 60 hours to decompress at the end of the project.

Two 90 mm diameter cores were drilled at the bottom to hold the bearing packs, and then two 250 mm diameter cores were drilled at the bottom corners, and joined using a push-cut diamond wire saw. The bearing packs (two steel tubes split down the middle and filled with steel balls) were inserted in the 90 mm holes, and then the top corners were cored, the sides were cut and then the top. (After this, the block rested on the bearing packs.) The block was cut with a two degree taper to ensure that it did not seize when being retracted, as the only clearance was the width of the wire cut.

The dimensions of the main frame were constrained by the size of the alcove (which was barely larger than the block being removed) and the need to provide sufficient clearance to pull the block into the frame, using an anchor in the block and a winch. This meant that the main frame was not sufficiently strong to allow it to be lifted from the top, with the block of concrete inside it. A special spreader was fabricated, and this was bolted through the side and into the base of the main frame. The divers attached the spreader, but the dimensions of the nut and bolt were such that the diver could do this using gloves.

All operations were tested on dry land before the rig was deployed to site, to ensure that they were viable.